In a case where an outside temperature is low, there is a problem that after the stop of a fuel cell system, water remaining in the system is frozen to break pipes, valves and the like. In view of such a problem, a method is suggested in which after the stop of the fuel cell system, scavenging process is performed to discharge, from the system, the water content accumulated in a fuel cell (see Japanese Patent Application Laid-Open No. 2005-141943).
The internal water content of the fuel cell has a correlation with the impedance of the fuel cell. At present, the impedance of the fuel cell is measured to indirectly grasp the water content in the fuel cell (Japanese Patent Application Laid-Open No. 2003-86220).